The present invention relates to an hydraulic apparatus for temporarily increasing the lifting capacity of an excavator.
Hydraulic excavators typically include a boom which is pivoted by a hoist cylinder with respect to the excavator chassis so that an outer portion of the boom moves in a vertical arc. A digging bucket is coupled to the boom, usually by means of an arm pivotably attached to the outer end of the boom. In normal operation the bucket, arm and boom are pivoted with respect to one another so that the bucket moves toward the excavator chassis to scoop up earth or other material.
Hydraulic excavators are also, occasionally, used to lift heavy objects such as sewer pipes, structural components or dirt shields. When the excavator is employed by this secondary lifting function, the excavator bucket is typically disabled. The lifting capacity of the excavator is limited principally by the maximum pressure applied to a hoist cylinder which pivots the excavator boom. This capacity may be insufficient to lift some heavy objects.
Various hydraulic control systems for excavators are known in the prior art. One such control system, shown in U.S. Pat. No. 3,865,013 to Mastaj, provides a relatively complex hydraulic interlock for a tractor mounted pavement breaker. The system prevents the hydraulic pavement breaker from being lifted off the work by a hydraulic boom of the tractor while the pavement breaker is being operated. However, pressure relief valves rather than interlock systems such as the Mastaj system are often employed to limit the operation of excavator hydraulic rams. A pressure relief valve system for use with a hoist or lifting cylinder of an excavator is disclosed, for example, in U.S. Pat. No. 3,324,881 to Keir.
An adjustment mechanism for the relief valve of the hoist cylinder of an excavator could be provided to permit the operator to selectively increase the maximum pressure applied to the hoist cylinder. Experience has indicated however, that operators typically run excavators at maximum capacity for as much of the time possible. It is expectable, then, that a readily adjustable relief valve for a hoist cylinder would be set to its upper pressure limit by the operator and would be left at that setting. As a consequence, the hoist cylinder would be operated at the increased capacity much of the time. As a general rule of thumb, an increase of ten percent in the maximum fluid pressure applied to a hydraulically actuated system in an excavator, cuts the lift expectancy of the system in half. Thus, the use of the hoist cylinder at an increased capacity during normal excavator operations will tend to shorten the life of the excavator and risks injury to personnel associated with the excavator should the excavator fail catastrophically.
A known apparatus for increasing the lifting capacity of an excavator provides for an increase in the maximum pressure applied to the hoist cylinder while reducing the volume output of the hydraulic system, thus reducing the speed of the rams powered by the hydraulic system. The apparatus has the disadvantage that the lifting speed of the excavator is reduced. Moreover, the excavator can be misused to perform slow digging with the bucket at increased hydraulic pressures. Thus, the excavator could, for example be used to slowly excavate large rocks which could not be moved if the excavator were operated at normal hydraulic pressures. Such operation could damage the excavator.
Accordingly, it is an object of the present invention to provide an apparatus for increasing the lifting capacity of an excavator for use in lifting heavy objects, while conjointly disabling the hydraulic circuit used for normal digging operations.
It is another object of the present invention to provide as easily and inexpensively fabricated apparatus for increasing the lifting capacity of an excavator while enabling the excavator boom to be pivoted at normal speed.
These and other objects and features of the invention will become apparent from the claims and from the following description when read in conjunction with the accompanying drawings.